Energy: The Ability to do Work

Work: transfer of energy by a force. Energy is transferred by a force applied over a distance. The climbers are transferring chemical energy into gravitational potential energy. We would say the work they do inclimbing = their gravitational potential energy at the top + thermal energy produced. Work is the name for the transfer of energy. When money is transferred we might call it "spending" or "deposit" but the transfer of energy is done by work. If force is a function of position, work is the integral of F "dot" dx. For a constant force this boils down to W = F x cos theta.

Kinetic Energy KE = ½ mv2 Energy of motion - moving mass can do work.

Gravitational Potential Energy Ug = mgh Energy of mass lifted in a gravitational field. Mass at this height will be pulled down by gravity and will do work.

Elastic Potential Energy Ue = ½ kx2 Energy of stretch. Proportional to k, the stiffness, and the amount of stretch, squared.

Electrical Energy Radiant Energy

Thermal Energy: A rise in temperature or change in chemical bonds Energy transfers are almost always "non-conservative", that is part the of the energy is transformed into thermal energy and is not useful for doing work anymore. Thermal energy can be used to do work, but it is inefficient - impossible to convert all thermal energy into mechanical energy. In physics world the production of thermal energy is ignored.

Contest: List several examples of energy in each category. Kinetic Energy: Energy of motion Potential Energy: Stored Radiant Energy: Radiated Thermal Energy: (Actually a type of kinetic energy)

Unit for Work, Heat, and Energy: Joule 1 Cal Diet Coke = 4000 Joules 1 Joule = 1 Newton meter 1 Newton = 1 kg m/s2

1 BTU = 1054Joules 1 gallon gas = 125,000 BTU

Eureka Movies (10 minutes): Work (8) Kinetic Energy (9)

Heat: transfer of thermal energy by radiation, convection, conduction.

Electricity is Kinetic Energy! Where does our electricity come from?

Transformed and Transferred Energy Radiant Nuclear Kinetic Chemical Chemical Thermal

Nuclear, coal, geothermal, natural gas – most of our energy “turns the crank”.

These turn the crank.

Solar Panels convert directly to electricity.

No energy converter is 100% efficient: Solar Panels: 4% efficiency in 1980 12% efficiency today 42% in lab Wind Energy: Theoretical max 57% Actual 30%? Conventional: 35-45% efficiency

Today Tomorrow Then… Conservation of Energy Finish Activity (Parts 2 and 3) Tomorrow Practice Conservation of Energy Then… Work Power

Conservation of Energy Within a system energy cannot be created or destroyed, only converted from one form to another. KEInitial + PEInitial = KEFinal + PEFinal

Conservation of Energy For our problems, we will ignore dissipated energy unless the problem says not to. (100% conversion) If the PE is GPE this is the equation… ½ mv2 + mgh = ½ mv2 + mgh

Problem #1 A 2 kg ball is at rest at the top of a 2 m tall ramp. It rolls to the bottom of the ramp. What is its velocity at the bottom of the ramp? ½ mv2 + mgh = ½ mv2 + mgh